Behaviour Of Unsaturated Soils Research Articles

Using the Continuously Disturbed Line to Estimate the Mechanical Behaviour of Unsaturated Compacted Soils

A simple technique which approximates the continuously disturbed line (CDL) using the relationship between water content and matric suction of compacted fine-grained soils, or apparent continuously disturbed line (CDLa) is presented. The CDL is defined as the relationship between a soils water content and suction reached by a soil specimen sheared under constant water content conditions (Croney and Coleman, in J Soil Sci 5(1):75–84, 1954). The critical state line (CSL), generated from specimens sheared under constant net normal stress, and the approximate continuously disturbed line (CDLa) appear to overlap at higher suction values. From plots of water content against matric suction, the CSL appears as a bi-linear relationship which lies below the CDLa with a shallower slope at lower matric suctions. The intersection point of the CSL and CDLa corresponds to the point where a unique value of the microstructural water content, e wm , can be back-calculated from the shear strength data. The relationship between the void ratio and the water content of compacted specimens provides an indication of the moisture content when the air entry value is reached.

Thermo-mechanical constitutive modeling of unsaturated clays based on the critical state concepts

A thermo-mechanical constitutive model for unsaturated clays is constructed based on the existing model for saturated clays originally proposed by the authors. The saturated clays model was formulated in the framework of critical state soil mechanics and modified Cam-clay model. The existing model has been generalized to simulate the experimentally observed behavior of unsaturated clays by introducing Bishop's stress and suction as independent stress parameters and modifying the hardening rule and yield criterion to take into account the role of suction. Also, according to previous studies, an increase in temperature causes a reduction in specific volume. A reduction in suction (wetting) for a given confining stress may induce an irreversible volumetric compression (collapse). Thus an increase in suction (drying) raises a specific volume i.e. the movement of normal consolidation line (NCL) to higher values of void ratio. However, some experimental data confirm the assumption that this reduction is dependent on the stress level of soil element. A generalized approach considering the effect of stress level on the magnitude of clays thermal dependency in compression plane is proposed in this study. The number of modeling parameters is kept to a minimum, and they all have clear physical interpretations, to facilitate the usefulness of model for practical applications. A step-by-step procedure used for parameter calibration is also described. The model is finally evaluated using a comprehensive set of experimental data for the thermo-mechanical behavior of unsaturated soils.

A bounding surface plasticity model for unsaturated soil at small strains

Most existing hydromechanical models for unsaturated soils are not able to fully capture the nonlinearity of stress–strain curves at small strains (less than 1%). They cannot therefore, for example, accurately predict ground movements and the performance of many earth structures under working conditions. To tackle this problem, a state-dependent bounding surface plasticity model has been newly developed. Particularly, the degradation of shear modulus with strain at small strains ranging from 0.001% to 1% is focused. The proposed model is formulated in terms of mean average skeleton stress, deviator stress, suction, specific volume and degree of saturation. Void ratio-dependent hydraulic hysteresis is coupled with the stress–strain behaviour. Different from other elastoplastic models for unsaturated soils, plastic strains are allowed inside bounding surfaces. In this paper, details of model formulations and calibration procedures of model parameters are presented. To evaluate the capability of the new model, it is applied to simulate a series of triaxial compression tests on compacted unsaturated silt at various suctions. Effects of suction, drying and wetting as well as net stress on unsaturated soil behaviour are well captured. The model shows good predictions of the degradation of shear modulus with strain over a wide range of strains from 0.001% to 1%. Copyright © 2015 John Wiley & Sons, Ltd.

Evolving Functional Expression of Permeability of Fly Ash by a New Evolutionary Approach

The influence of stress, which is one of the constitutive variables that governs unsaturated soil behavior, on the permeability has been recognized by various researchers. Stress factor is essential to study as it drastically alters the soil matrix which includes macropores, minipores and micropores and thus affecting the ability of soils to retain water and also permeability. An evolutionary approach of multi-gene genetic programming (MGGP), which automatically evolves model structure and coefficients can also be applied. However, the effective functioning of MGGP may be affected by the formulation of robust-multi-gene model and the poor selection of the best model. Therefore, a new evolutionary approach of MGGP (E-MGGP) is proposed by incorporating the stepwise and the classification strategies and applied to formulate the functional relationship between the permeability and input variables (stress and initial void ratio). The results reveal that the E-MGGP model outperformed the other three models (MGGP, support vector regression and artificial neural network). Further, the parametric and sensitivity analysis conducted validates the robustness of our proposed model by unveiling dominant input process variables and hidden nonlinear relationships.

Use of Effective Stresses to Model the Collapse upon Wetting in Unsaturated Soils

AbstractThis paper presents the modeling of the phenomenon of collapse upon wetting using the effective stress approach. To this purpose, Bishop’s effective stress equation is applied in addition to an elastoplastic framework for the volumetric behavior of unsaturated soils. Bishop’s parameter χ is obtained from a porous-solid model that is able to simulate the soil-water retention curves during wetting-drying cycles. The proposed framework includes the hysteresis of the retention curve and to some extent the hydro-mechanical coupling of unsaturated soils. This model is able to reproduce quite precisely the phenomenon of collapse upon wetting using a single valued compressibility index. It also represents a unifying framework for saturated and unsaturated soils.

Modified Unconfined Compression Testing System to Characterize Stress–Strain Behavior of Unsaturated Soils at Low Confining Stresses

In geotechnical and pavement engineering, many projects involve shallow, unsaturated soil with low confining stress. Suction-controlled triaxial tests usually are used to investigate stress–strain behavior (i.e., deformation and strength) in such soils. However, because they require significant modifications to the conventional apparatus used to test saturated soils and are time-consuming to perform, the tests cannot be justified for time-sensitive engineering projects. A modified unconfined compression testing system is presented to investigate the stress–strain behavior of unsaturated soils at low confining stresses. Negative air pressure (i.e., vacuum) provides the low confining pressure required for the tests, and high-suction tensiometers are used to monitor variations in the soil matric suction during testing. A photogrammetric method is used to reconstruct the three-dimensional model of unsaturated soil specimens, from which total and localized volume changes are calculated. Methods are proposed for unsaturated shear strength analysis. A series of tests was carried out to demonstrate the use of the modified unconfined compression testing system to evaluate the stress-strain behavior of unsaturated soil under different confining stresses. Results indicated that the newly developed modified unconfined compression testing system was a cost-effective method for the rapid evaluation of stress–strain behavior of unsaturated soils under low confining stresses.

Effect of coarse content on shear behavior of unsaturated coarse granular soils

The effects of coarse content on the microstructure, soil-water characteristics, shearing behavior, and critical state characteristics of three types of unsaturated coarse granular soil with different coarse contents are investigated in an integrated manner through an extensive laboratory test program. The tests were performed on samples with the same void ratio of 0.62. As the coarse content increases, the pore-size distribution of the soil changes from unimodal to bimodal. Accordingly, the soil-water characteristic curve (SWCC) changes from unimodal to bimodal. The whole drainage process of the bimodal SWCC is divided into four stages based on the role of the pore water. The contribution of suction to the peak state parameters and critical state parameters, except for the intercept of the critical state line in the stress plane (μ(s)) of a bimodal soil, reaches its maximum in the first drainage region. The pore water in the two pore series of a bimodal soil contributes to the shear strength differently during the desaturation process. Hence, the variations of the maximum dilatancy and the shear strength parameters with degree of saturation are highly nonlinear and μ(s) shows bimodal features. The contributions of suction to the shear strength parameters decrease with increasing coarse content.

Relative performance of two unsaturated soil models using different constitutive variables

Mechanical and water retention behaviour of unsaturated soils is investigated in the context of two well established coupled constitutive models, each of which is formulated in terms of a different set of stress state variables or constitutive variables. Incremental relationships describing the volume change and variation of the degree of saturation are derived for each model. These incremental relationships are used to simulate a set of experimental tests on compacted Speswhite kaolin previously reported in the literature. Six individual tests, involving isotropic compression and various forms of shearing, are analyzed in the context of the incremental forms developed, and the model predictions are then compared against experimental results. The results show that, although each constitutive model uses a different set of constitutive variables and a different scheme for coupling mechanical and water retention behaviour, the two sets of model predictions are similar and both sets provide a reasonable match to the experimental results, suggesting that both models are able to capture the relevant features of unsaturated soil behaviour, despite expressing the constitutive laws in different ways.

Soil–Water Characteristic Curve of Gypseous Soil

The soil–water characteristic curve (SWCC), also known as soil–water retention curve, is a key tool in assessing the behavior and properties of unsaturated soil. The SWCC of gypseous soil with 0, 5, 15 and 25 % gypsum content was determined, using tensiometric plate, osmotic membrane and vapour equilibrium techniques, with suction pressures ranging between 10 and 1,000,000 kPa. The effect of two compaction efforts, standard and modified, was examined on the SWCC of soil samples. The water-holding capacity of soil samples increased with increasing gypsum content and applied compaction effort. Mercury porosimetry tests and scanning electron microscope images revealed that compaction and the presence of gypsum increased the number of capillary pores. These changes in the pore size distribution of soil samples induced modifications in the volumetric water content at air-entry value of the tested samples. All experimental SWCC data were fitted using the Fredlund and Xing (Can Geotech J 31(4):521–532, 1994) and Van Genuchten (Soil Sci Soc Am J 44:892–898, 1980) models. Results showed that a high coefficient of determination (R2) can be achieved by using both models to fit the experimental results of gypseous soil SWCC.

An evolutionary approach to modelling the thermomechanical behaviour of unsaturated soils

SummaryA new data‐mining approach is presented for modelling of the stress–strain and volume change behaviour of unsaturated soils considering temperature effects. The proposed approach is based on the evolutionary polynomial regression (EPR), which unlike some other data‐mining techniques, generates a transparent and structured representation of the behaviour of systems directly from raw experimental (or field) data. The proposed methodology can operate on large quantities of data in order to capture nonlinear and complex relationships between contributing variables. The developed models allow the user to gain a clear insight into the behaviour of the system. Unsaturated triaxial test data from the literature were used for development and verification of EPR models. The developed models were also used (in a coupled manner) to produce the entire stress path of triaxial tests. Comparison of the EPR model predictions with the experimental data revealed the robustness and capability of the proposed methodology in capturing and reproducing the constitutive thermomechanical behaviour of unsaturated soils. More importantly, the capability of the developed models in accurately generalizing the predictions to unseen data cases was illustrated. The results of a sensitivity analysis showed that the models developed from data are able to capture and represent the physical aspects of the unsaturated soil behaviour accurately. The merits and advantages of the proposed methodology are also discussed. Copyright © 2014 John Wiley & Sons, Ltd.

An advanced hydro-mechanical constitutive model for unsaturated soils with different initial densities

This paper presents an advanced constitutive model for unsaturated soils, using Bishop’s effective stress (σ′) and the effective degree of saturation (Se) as two fundamental constitutive variables in the proposed constitutive model. A sub-loading surface and a unified hardening parameter (H) are introduced into the σ′–Se modelling framework to interpret the effects of initial density on coupled hydro-mechanical behaviour of compacted soils. Compared with existing models in the literature, the main advantage of the proposed model that it is capable of modelling hydro-mechanical behaviour of unsaturated soils compacted to different initial densities, such as the dependence of loading–collapse volume on initial void ratio and density effect on the shearing-induced saturation change. The proposed model requires 13 material parameters, all of which can be calibrated through conventional laboratory tests. Numerical studies are conducted to assess the performance of the model for a hypothetical soil under two typical hydro-mechanical loading scenarios. The proposed advanced unsaturated soil model is then validated against a number of experimental results for both isotropic and triaxial conditions reported in the literature.

Hydromechanical Behaviors of Unsaturated Soils

AbstractIn this paper, the hydromechanical behavior of unsaturated soils is explored. A constitutive model is proposed to describe the coupled hydraulic and mechanical behavior of unsaturated soils. The compression stiffness parameters, such as λ(s), κ(s), λs, and κs, are analyzed, taking into account the influences of suction, degree of saturation, and net mean stress. Based on the volume change behavior under constant suction and the water retention properties, the relationship between the degree of saturation and volume change has been investigated under both net mean stress and suction. The proposed model is capable of describing the mechanical behavior of unsaturated soils for either isotropic compression or triaxial compression. Indeed, the simulation results of three cases of suction-controlled laboratory tests show good agreement with experimental data.

Coupled Thermal Conductivity Dryout Curve and Soil–Water Characteristic Curve in Modeling of Shallow Horizontal Geothermal Ground Loops

Shallow horizontal ground loops harness stored subsurface thermal energy and operate in conditions where the moisture and temperature of the surrounding soil vary spatially and temporally. The thermal conductivity of the soil is dependent on soil moisture and temperature and is a design parameter that greatly influences the size and performance of horizontal ground loops. However, soil thermal conductivity is often assumed to be constant and conservative estimates are used in the design of ground loops. Two fundamental constitutive relationships, the thermal conductivity dryout curve (TCDC) and the soil–water characteristic curve (SWCC), can be coupled and used to quantify transient moisture-dependent thermal behavior of soil. In this study, coupled TCDCs and SWCCs were utilized in two-dimensional models based on the finite-element method to predict moisture migration effects on transient hydraulic and thermal behavior of unsaturated soil surrounding geothermal exchange loops. Soil thermal conductivity predicted from using coupled TCDCs and SWCCs are compared to the conventional method of using a conservative value. Results suggest that employing coupled TCDCs and SWCCs can provide realistic and improved values of soil thermal conductivity for the design of horizontal ground loops.

A Theoretical Framework for Modeling the Chemomechanical Behavior of Unsaturated Soils

A theoretical framework is presented for modeling the chemomechanical behavior of multiphase porous media, in general, and unsaturated soils, in particular, which can address skeletal deformation, fluid flow, heat conduction, solute diffusion, chemical reaction, and phase transition in a consistent and systematic way. A general expression is derived for the electrochemical potential of a fluid species with explicitly accounting for the effects of osmosis, capillarity, and adsorption. The equilibrium behavior of porous media is investigated, and the composition of pore water pressure is identified. Explicit formulations are developed for the effective stress and intergranular stress, with consideration of physicochemical effects. It is shown that the negative water pressure measured by a conventional transducer can be significantly different than the true pore water pressure. It is also theoretically revealed that, other than the soil water characteristic function, a new pressure (or potential) function accounting for the physicochemical effects is generally required in analyzing the coupled chemomechanical processes in unsaturated soils. The new theory is capable of effectively explaining many salient phenomena occurring in water‐saturated porous media with a degree of saturation varying from an extremely low value to 100%, including Donnan osmosis, capillary fringe, air entry value, initial hydraulic head during seepage, and pressure solution. The new theory can be used to analyze the multiple coupled physical and chemical processes in the vadose zone.

Semi-analytical solution to one-dimensional consolidation for viscoelastic unsaturated soils

This paper presents a semi-analytical solution to one-dimensional consolidation of viscoelastic unsaturated soils with a finite thickness under oedometric conditions and subjected to a sudden loading. The solution is obtained by using Lee’s correspondence principle based on the semi-analytical solution to one-dimensional consolidation of elastic unsaturated soils. The boundary contains the top surface permeable to water and air and the bottom impermeable to water and air. A typical example is given to show the evolution of excess pore-air and pore-water pressures as well as the total degree of consolidation of the soil layer with time for different ratios of air–water permeability coefficient, elastic modulus and viscoelastic coefficient. The one-dimensional consolidation behavior of viscoelastic unsaturated soil is discussed according to the semi-analytical solution. These results contribute to a better understanding of the consolidation behavior of viscoelastic unsaturated soils.

Effects of Constraints on van Genuchten Parameters for Modeling Soil-Water Characteristic Curves

Water-retention data for a suite of predominantly sandy soils were analyzed to quantify the effects of two constraints commonly applied to the three-parameter model for the soil-water characteristic curve (SWCC). Systematic effects were observed. Constraining either the symmetry parameter (m) to m=1−1/n or residual saturation (Sr) to zero resulted in values for the air-entry parameter α that were approximately 25–30% different than if no constraints were applied. The m=1−1/n constraint forced the pore-size distribution parameter n to fall within a narrow range (∼1.5<n<∼4.0), whereas n values using independent m and n were more variable and could be as large as 35. The m=1−1/n constraint produced best-fit residual saturation values that were approximately 0.02 higher than without the constraint. These differences are significant and should be considered when adopting existing or new pedotransfer functions for estimating water-retention behavior of unsaturated soils from more easily measured properties.

Implicit scheme for integrating constitutive model of unsaturated soils with coupling hydraulic and mechanical behavior

A constitutive model of unsaturated soils with coupling capillary hysteresis and skeleton deformation is developed and implemented in a fully coupled transient hydro-mechanical finite-element model (computer code U-DYSAC2). The obtained results are compared with experimental results, showing that the proposed constitutive model can simulate the main mechanical and hydraulic behavior of unsaturated soils in a unified framework. The non-linearity of the soil-water characteristic relation is treated in a similar way of elastoplasticity. Two constitutive relations are integrated by an implicit return-mapping scheme similar to that developed for saturated soils. A consistent tangential modulus is derived to preserve the asymptotic rate of the quadratic convergence of Newton’s iteration. Combined with the integration of the constitutive model, a complete finite-element formulation of coupling hydro-mechanical problems for unsaturated soils is presented. A number of practical problems with different given initial and boundary conditions are analyzed to illustrate the performance and capabilities of the finite-element model.

Cyclic behaviour of an unsaturated silt at various suctions and temperatures

Subgrade soils are constantly subjected to daily variations of temperature and suction in the field. Their deformation under cyclic traffic loads can greatly affect the performance of a pavement. Yet, the behaviour of unsaturated subgrade soils under cyclic loads at various temperatures and suctions is not fully understood. In this study, a new suction- and temperature-controlled cyclic triaxial apparatus was developed. Using temperature-compensated Hall effect transducers, the accumulated plastic axial and volumetric strains and resilient modulus of an unsaturated silt under cyclic loads were measured at suctions ranging from 0 to 60 kPa and temperatures ranging from 20 to 60°C. Experimental results show that yield stress increases with increasing suction (suction-induced hardening), but decreases with increasing temperature (thermal-induced softening). Consequently, axial plastic strain accumulated during cyclic loading–unloading is larger at lower suctions and higher temperatures. At a given temperature, measured resilient modulus significantly increases when suction increases. On the other hand, the influence of temperature on resilient modulus depends on suction level. At zero suction, resilient modulus is slightly affected by temperature. At suction larger than zero, an increase in temperature induces an obvious decrease in resilient modulus. It is evident that thermal effects on resilient modulus are more significant at unsaturated than saturated states.

New thermal triaxial apparatus for unsaturated soils using the osmotic method

The study of the temperature effects on the behaviour of saturated and unsaturated soils on triaxial paths requires the development of a new triaxial experimental device of 3.5 MPa of confining pressure and equipped with a heating collar-controlled temperature. In addition to this case of the unsaturated soils, a special pedestal is developed, so as to impose suctions up to 9 MPa using an osmotic solution of polyethylene glycol (PEG) 6000 through a permeable semi-membrane. Measurements of the degree of saturation and the variation of volume during tests induce various problems, primarily related to the thermal dilation of the osmotic solution and the cell-water system. These problems have been solved by carrying out a preliminary calibration. The applicability of this new apparatus concerns environmental geotechnics and geological engineering such as the design and behaviour of engineered barriers as well as the behaviour of the receiving layers of storage sites like radioactive waste.

Formulation of thermo-hydro-mechanical coupling behavior of unsaturated soils based on hybrid mixture theory

Thermo-Hydro-Mechanical (THM) coupling processes in unsaturated soils are very important in both theoretical researches and engineering applications. A coupled formulation based on hybrid mixture theory is derived to model the THM coupling behavior of unsaturated soils. The free-energy and dissipative functions for different phases are derived from Taylor’s series expansions. Constitutive relations for THM coupled behaviors of unsaturated soils, which include deformation, entropy change, fluid flow, heat conduction, and dynamic compatibility conditions on the interfaces, are then established. The number of field equations is shown to be equal to the number of unknown variables; thus, a closure of this coupling problem is established. In addition to modifications of the physical conservation equations with coupling effect terms, the constitutive equations, which consider the coupling between elastoplastic deformation of the soil skeleton, fluid flow, and heat transfer, are also derived.